Contemporary portable electronic appliances rely almost exclusively on rechargeable lithium (Li)-ion batteries as the source of power. In a typical Li-ion battery, the cell includes metal oxides for the positive electrode (or cathode), carbon/graphite for the negative electrode (or anode), and a lithium salt in an organic solvent for the electrolyte. More recently, lithium metal phosphates have been used as a cathode electroactive material. Lithium iron phosphate is now recognized as a safe and reliable cathode material for secondary batteries. It is a next-generation replacement for the more hazardous lithium cobalt oxide that is currently used in commercial lithium ion batteries.
Li-ion batteries using lithium iron phosphate (LFP)-based cathode materials are currently found in cordless hand tools and on-board UPS devices. Battery packs have recently been demonstrated for transportation including aviation and rechargeable electric vehicle (REV), plug-in hybrid electric vehicle (PHEV) automobiles and buses.
In order to find more wide-spread application of Li-ion batteries for the transportation industry, an efficient manufacturing process of the nano-phosphate cathode material is needed. Iron (III) phosphate (also referred to as ferric phosphate or iron phosphate) is used in several structural forms as raw material for the LFP production process. In some embodiments, the form is crystalline ferric phosphate dihydrate, which has three related crystal phases, Strengite, Metastrengite I and Metastrengite II. Currently, ferric iron phosphate dihydrate is produced by several commodity bulk suppliers mainly as a food additive, a colorant in ceramic processes, or other purposes using processes generally involving a one-pot oxidation of a iron (II) compound. The iron phosphate produced by this method (or variants) usually has inferior qualities and includes impurities, thus making it less desirable as a precursor for the synthesis of high-quality nano-phosphate LFP. In addition, the characteristics of iron (III) phosphate desired for the synthesis of LFP in lithium ion battery may be very different from those for other intended uses. For example, one requirement for the use of iron phosphate as a nutrient supplement or food additive is that the chemical not contain toxic levels of certain metal contaminants like lead, mercury or chromium-6. In contrast, toxicity is not an important requirement for iron phosphate as a precursor to lithium ion battery application. Similarly, the purity and consistency of ferric phosphate color is a desired characteristic for materials used in the manufacture of ceramics. However, color purity is not primary concern for the synthesis of LFP cathode material for lithium ion battery. Yet another example is the phosphorus to iron ratio. In the animal feed stock application of iron phosphate, the exact ratio of Fe/P is not critical but higher Fe/P ratios is desirable. Thus, the Fe/P ratio may not be carefully controlled and in some instances higher phosphorous content is desired.
The ferric phosphates available from many commodity bulk chemical suppliers (Noah Chemical, Strem Chemical, Alpha Aesar Chemical etc.) contain relatively high levels of sulfate or nitrate anion (0.1-2 wt % impurity anion), as demonstrated by chemical analysis of the ferric phosphates available from these suppliers. This suggests that a common route used by these suppliers to produce ferric phosphate involves a iron salt solution, most commonly ferrous sulfate or ferric nitrate. Ferric phosphate with a high level of anion impurity is not suitable for a raw material for the manufacture of lithium iron phosphate for the use in lithium ion batteries because the impurities are susceptible to reactions that are detrimental to battery operation and/or do not support the electrochemical cycling of the cell.
Other synthetic methods for synthesizing ferric phosphate use iron oxide (Fe (II), Fe (III) or Fe (II/III)), iron hydroxide, iron carbonate, or a combination thereof, as the starting iron material. In such methods the iron oxide or carbonate is reacted with a dilute phosphoric acid solution and an oxidizing agent (if necessary) and heated to produce the crystalline ferric phosphate compound. Despite such recent attempt to improve the quality of ferric phosphate product, commercially available ferric phosphate is still not optimized for the use in lithium ion battery.
Therefore, because the desirable characteristics of iron phosphate precursor for the synthesis of battery-grade LFP are often different from or in contradiction with those for other intended uses, the existing commodity chemical market is an impractical source for iron phosphate as LFP synthesis precursor. In addition, iron phosphate from commercial sources synthesized by the existing one-pot method often has impurities, e.g., sulfate, chloride, and nitrate, which are detrimental to lithium ion batteries. Furthermore, different batches of commercially available iron phosphate material often have inconsistent properties. Thus, there remains a need for the development of a new synthetic method for producing highly pure iron phosphate with consistent and desirable properties for the synthesis of battery-grade LFP.